This invention relates to an improved laminated wood flooring for van-type truck trailers. Fiber reinforced polymeric composite materials are used in conjunction with edge-glued laminated wood flooring to provide improved mechanical properties, moisture protection, fatigue resistance and light weight.
Conventional wood flooring for over-the-road truck trailers is normally manufactured with hardwoods such as oak, maple, birch, beech, etc. The green lumber used as a starting material in such manufacture is suitably dried in special drying chambers under controlled conditions. The dried lumber is then sawed into strips of rectangular cross-section and defective portions are eliminated by cross cutting the strips. During the cross-cutting process, xe2x80x9chooksxe2x80x9d are formed at the ends of the lumber strips. The relatively defect-free lumber strips are smeared on their vertical sides or edges with an adhesive such as urea-melamine formaldehyde or polyvinyl acetate. The uncured edge-glued lumber strips are then assembled on a conveyor by placing them side to side and behind other strips which were previously assembled. The adhesive is cured by applying heat and edge pressure to large sections of the assembled lumber strips thus forming a unitary panel. During the assembly of the lumber strips, xe2x80x9chook jointsxe2x80x9d are formed at each end of every strip. These joints are simple mechanical couplings with no significant adhesive bonding. Often times, due to imperfect assembly, a readily visible gap is formed at the hook joints which can be seen from the top and bottom surfaces of the completed laminated wood floor.
The cured laminated wood is cut to a desired length (up to about 60 feet) and width (about 6 to 18 inches) to form boards. The boards are then planed to a desired thickness and shiplaps and crusher beads are machined on its sides. A shiplap is a rectangular projecting lip running along the length on each side of a floor board. The crusher bead is a small semi-circular projection running along the length on each side of a board and placed over or below a lip. When the floor boards are assembled in a trailer such that the side edges of corresponding boards are squeezed together, the shiplaps of adjacent boards overlap to form a seam. The crusher beads provide spacing between adjacent boards and help in preventing buckling of the boards due to expansion on wetting. A wood putty is applied at the hook joints on the top and bottom surfaces of the boards to fill any resident gaps. Finally, the underside of the floor boards are coated with a polymeric substance termed as xe2x80x9cundercoatingxe2x80x9d to provide moisture protection. The finished floor boards are assembled into a kit of about eight boards for installation in trailers. Normally, a kit consists of two boards with special shiplaps so that they will fit along the road and curb sides of a trailer. The other boards may be identical in design and they are placed between the road and curb side boards. All the boards are supported by thin-walled cross-members of I, C or hat sections, each having an upper flange, which span the width of the trailer and are regularly spaced along the length of the trailer. Each floor board is secured to the cross-members by screws extending through the thicknesses of the board and the upper flanges of the cross-members.
Hardwood-based laminated wood flooring is popularly used in truck trailers since it offers many advantages. The surface characteristics of hardwoods such as high wear resistance and slip resistance are most desirable. The strength and stiffness of the flooring is important for efficient and safe transfer of the applied loads to the cross-members of the trailer. The shock resistance of wood is useful to withstand any sudden dropping of heavy cargo on the floor. Nail holding capability and ability to absorb small amounts of water, oil or grease without significantly affecting slip resistance are yet additional favorable properties of hardwood flooring.
Although the conventional hardwood flooring has many desirable features, it also suffers from certain disadvantages. For example, water from the roads is known to leak into trailers through the gaps of the hook joints that exist in the flooring. The reasons for the water leaks are believed to be the capillary action of the gaps and the tendency of the end grain of wood to absorb and store water. Although the undercoating is supposed to provide a barrier to the path of water, it may not properly cover larger gaps thus exposing them to moisture. Wetting and drying cycles can degrade the undercoating leading to its cracking and peeling away from the wood. Bending of the floor between two adjacent cross-members due to any applied load on the top of the floor also has a tendency to open the hook joints and enlarge the gaps.
A lift truck is often used on the trailer floor to load and unload cargo. The dynamic action of a moving lift truck placing heavy cargo on the trailer floor creates severe stress concentration in the flooring and some of the cross-members. A very large proportion of the weight of the lift truck and that of the cargo is transferred to the flooring through the wheels of the front axle of the lift truck due to the momentary raising of the rear axle when the lift truck is dynamically placing a heavy cargo on the floor. The effect of repeated lift truck operation on the conventional wood floor causes considerable fatigue damage including: delamination of the edge glue lines near the hook joints leading to the xe2x80x9cpop-outxe2x80x9d of the lumber strips; crack initiation and propagation in wood on the underside of the floor due to longitudinal tensile stresses; and edge glue line delamination due to shearing, transverse bending and twisting of the floor. The combination of moisture attack and fatigue damage to the wood floor shortens its life thus necessitating its replacement. In some cases, catastrophic structural failure of the trailer flooring system may occur leading to the unacceptable injury of working personnel and damage to machinery.
To alleviate the above-mentioned problems, a novel fiber reinforced composite wood flooring was designed, tested and refined to be an improvement over conventional wood flooring. This new composite wood flooring consists of conventional laminated wood with an underlay of fiber reinforced plastic (FRP). The top surface of the composite wood flooring is essentially the same as that of the conventional wood flooring. Since the FRP is impervious to the passage of water, it completely seals the bottom of the wood floor and solves the problem of leaky hook joints. The fiber reinforcement improves the mechanical properties of the flooring and therefore the thickness of the laminated wood can be reduced. Thus, a thinner and lighter composite wood flooring can be produced with equivalent strength when compared to a thicker conventional wood flooring. Since the reinforcement provides an excellent barrier to the xe2x80x9cpop-outxe2x80x9d of lumber strips, the fatigue resistance of the composite wood flooring can be improved over that of the conventional wood flooring.
Technologists are constantly trying to find ways to improve the mechanical properties, reduce weight and improve moisture resistance of wood flooring. Fouquet, U.S. Pat. No. 5,143,418 describes the use of composite plywood panels as flooring in truck trailers. The plywood was composed of veneers of wood with a majority of the veneers oriented with the wood grain along the longitudinal direction while the remaining veneers were oriented with the wood grain along the perpendicular direction. The top and bottom surfaces of the plywood panels were overlaid with resin impregnated cellulose sheets for providing moisture and slip resistance. Clearly, Fouquet has not considered a floor design involving the FRP to provide higher strength and moisture protection.
Another area of related art is the use of FRP to improve the mechanical properties of structural wood members, such as beams, columns and trusses. Theakston (Canadian Agricultural Engineering, January 1965, Pages 17-19) has discussed the use of glass fibers and epoxy resin to reinforce laminated timber beams and arches. Triantafillou and Deskovic (Journal of Structural Engineering, Vol. 118, No. 5, May 1992, Pages 1270-1284) have published test results on the reinforcement of structural wood beams by adhesively bonding prestressed carbon fiber based FRP panels using epoxy adhesive. Thus the concept of reinforcing structural wood members (especially beams) with FRP has been known for over two decades. Tingley, U.S. Pat. No. 5,362,545 describes the use of a resorcinol adhesive to bond certain special composite panels to glue-laminated wood beams (Glulams). The special composite panels containing an aramid (Kevlar(copyright)) fiber reinforcement are abraded by sanding prior to bonding. The sanding process makes the panel xe2x80x9chair upxe2x80x9d due to Kevlar and helps to obtain improved bonding with wood. The Tingley patent teaches the utility of Kevlar in FRP panels to improve the bond strength of the FRP to wood while using a resorcinol adhesive.
The above-referenced patents and publications have not addressed the construction and related benefits of reinforced laminated wood flooring for use in truck trailers. The advantages of reinforcing the bottom side and disadvantages of reinforcing the top side of the laminated wood boards are not disclosed in these references. The publications do not discuss the remedies for the problems associated with the conventional wood flooring such as water leakage through the hook joints and fatigue damage due to lift truck traffic on the floor. There is no discussion in the publications regarding the type of reinforcements and resins that are suited for the fabrication of reinforced composite wood flooring. For example, the question of whether fiber reinforcement along the width (transverse) direction of the floor is advantageous is not addressed in these publications. A reinforced composite wood flooring construction such as that provided by the present invention which is suitable for lift truck movement and also for carrying cargo in a trailer has never before been invented. Methods of manufacturing the reinforced composite wood flooring of the invention have never been considered.
Unlike the FRP-wood structural beams, the reinforced composite wood flooring of the present invention is designed to provide several desirable features. The top side of the floor is simply composed of planed laminated wood as in the conventional wood flooring of trailer and truck beds. This provides a high coefficient of friction and slip resistance, thereby facilitating the safe movement of man and machine. The surface of wood can also absorb small amounts of water that may spill over it. Any oil or grease that spills on the wood surface can be wiped off and the left over slick is absorbed by the pores of wood as in a conventional wood flooring. On the contrary, even an extremely thin coating of oil on FRP can render it dangerously slippery and therefore, FRP is not laid on the top side of the composite wood floor. An FRP is laid on the bottom side of the laminated wood floor to provide protection to wood from the outside environment. The FRP underlay in the composite wood flooring eliminates the need for a polymeric undercoating and for puttying of hook joints that may be required in a conventional wood flooring. In fact, the need for hook joints themselves can be eliminated, that is, the lumber strips can be joined at butt ends thereof or by finger or lap and gap joints. However, the use of hook joints is preferred. The gaps in the hook joints are completely sealed by the FRP, thus preventing water leakage into the trailer through the flooring. Preferably, the FRP is fabricated with glass fibers and an epoxy resin. Other reinforcements such as carbon fiber and other thermosetting resins such as vinyl ester, polyester, phenolic resins and the like, and other thermoplastic resins such as polypropylene and polyamide resins and the like can also be used to fabricate the FRP. The glass fiber reinforcement provides an economical means of increasing the strength of the flooring in the longitudinal and transverse directions. The epoxy resin binds the glass fibers together and protects the fibers from adverse environment. The glass fibers resist the pop-out of the lumber strips during fatigue tests under simulated lift truck loading of a composite wood floor installed in a trailer. The transverse reinforcement resists the splitting of the FRP and delamination of edge-glue lines in laminated wood due to shearing, twisting and transverse bending loads on the floor. Thus, our design of the composite wood floor provides improvements in the prevention of moisture leakage through hook joints, maintains the desirable surface characteristics of the wood on the top surface and also provides improvements in fatigue resistance at lower weight.
The manufacture of the composite wood flooring can be accomplished by means of a suitable process wherein the fiber reinforced plastic is bonded to the surface of laminated wood. The fiber reinforcement is saturated with the resin and excess resin is squeezed out by passing the reinforcement between squeeze rollers. The resin-wetted reinforcement is placed in contact with the surface of the laminated wood and the resin is cured under heat and pressure. A batch-type or continuous hotpress can be used to apply heat and pressure on the resin-wetted reinforcement while it is still in contact with the laminated wood. A batch-type hotpress can be designed with a stationary hot platen on which the resin-wetted reinforcements are placed. Laminated wood boards are placed on top of the reinforcements. Air bladders are stationed above the hot platen to apply pressure on the top surface of the laminated wood. In a continuous hotpress, a series of hot rollers placed directly above a set of cold rollers can be used to apply heat and pressure on the composite material. The resin-wetted reinforcement is laid on laminated wood and then the combination is passed through the gap between the hot and cold rollers. At one end, the laminated wood is pushed into the roller hotpress while at the other end, the cured composite floor is pulled out of the roller hotpress.
The composite wood floor can also be manufactured by adapting the pultrusion process which normally involves the pulling of resin-wetted reinforcement through a stationary heated die where the FRP is shaped and the resin is cured. The laminated wood and the resin-wetted reinforcement can be pulled together through a heated die to produce composite wood flooring. Alternatively, a suitable prefabricated FRP sheet can be bonded to laminated wood using thermosetting or thermoplastic adhesives. The FRP sheet can be produced by pultrusion and continuous lamination processes. In the pultrusion process, the resin wetted reinforcements are pulled through a stationary heated die where the FRP is shaped and cured. In continuous lamination process, reinforcements are wetted with a polymeric resin. The wetted reinforcements are aligned in a die and then cured in an oven. Typical reinforcements used for the pultrusion process include continuous rovings, stitched, woven or knitted fabrics and continuous strand mats. In the continuous lamination process, chopped strands and chopped strand mats can be used in addition to the above-mentioned reinforcements. Preferred reinforcements for our prefabricated FRP are continuous rovings in the longitudinal direction of the FRP sheet and fabrics for transverse reinforcement. The fabrics may also be designed to provide bidirectional structural properties.
The present invention provides a novel composite wood flooring for use in truck trailers that is subjected to lift truck traffic on the top side and water spray on the bottom side. The composite wood flooring consists of conventional laminated wood to which a fiber reinforced plastic polymer is adhesively bonded on the bottom side. Since the top side is composed of wood, many desirable features of wood such as slip resistance, abrasion resistance and nailing capability are preserved. The underlayer of the composite wood flooring provides improved fatigue strength and moisture protection to wood. Since the mechanical properties of the reinforced composite wood flooring are superior to those of the conventional wood flooring, the thickness of the composite wood floor can be reduced leading to lower floor weight in a trailer. Alternatively, the load carrying capacity of the trailer can be increased with a suitable composite floor while having little or no increase in the weight of the trailer.